Metal forming method and apparatus

ABSTRACT

The specification and drawings disclose a method and apparatus for forming dish-shaped metal elements through the use of a combined fluid pressure and sheet tensioning process. The disclosed method is particularly intended for forming dish-shaped sheet metal elements having a length substantially greater than their width and includes applying a longitudinal tension to the sheet while permitting the lateral edges to move toward one another substantially unconstrained while the fluid pressure acts to bow the sheet. The disclosed apparatus includes a flexible frame of generally eliptical shape which clamps to the edge of the sheet. The frame is arranged so that as the sheet bows outwardly, the lateral sides of the frame are pulled inwardly driving the ends of the frame apart.

United States Patent [191 Swenson [451 Mar. 12, 1974 Paul F. Swenson, Cleveland, Ohio.

[73] Assignee: Great Lakes Sports Mfg. C0.,

Cleveland, Ohio [22] Filed: Jan. 29, 1973 [21] Appl. No.: 327,525

7 Related U.S. Application Data [62] Division of Ser. No. 176,519. Aug. 31, 1971. Pat. No.

[75] Inventor:

FOREIGN PATENTS OR APPLICATIONS 803,310 6/1936 France 72/353 Primary Examine'rRichard .1. Herbst Attorney, Agent, or FirmFay, Sharpe and Mulholland 57] ABSTRACT The specification and drawings disclose a method and apparatus for forming dish-shaped metal elements through the use of a combined fluid pressure and sheet tensioning process. The disclosed method is particularly intended for forming dish-shaped sheet metal elements having a length substantially greater than their width and includes applying a longitudinal tension to the sheet while permitting the lateral edges to move toward one another substantially unconstrained while the fluid pressure acts to bow the sheet. The disclosed apparatus includes a flexible frame of generally eliptical shape which clamps to the edge of the sheet. The frame is arranged so that as the sheet bows outwardly, the lateral sides of the frame are pulled inwardly driving the ends of the frame apart.

4 Claims, 10 Drawing Figures PATENTEB m 12 I974 SHEET 3 BF 3.

VI. 4 B

WW NN METAL FORMING METHOD AND APPARATUS This is a division of application Ser. No. 176,519,

filed Aug. 31, 1971, now US. Pat. No. 3,742,745.

The. subject invention is directed towardthe art of -metal forming and, more particularly, to an improved method and apparatus for stretch forming metal sheet.

The invention is especially suited for formingboat hulls and will be described with particular reference thereto; however, as will become apparent, the invention is capable of much broader application and could be employed for forming many different structures.

Typically, sheet metal elements and structures having compound curvatures must be formed by drawing operations. The die costs for such operations vary, of course, depending upon the size of the part, the amount of curvature, the depth of the draw, etc. Even for relatively small parts with simple, shallow draws, the die cost is substantial; However, when dealing with large, deep draw components such as boat hulls, fuel tanks or the like, the dieand press costs become extremely high and can only be justified for very large production runs. For relatively small runs, other fabricating techniques involving much more extensive use of manual labor will prove to be'less costly to employ.

Techniques for forming sheet metal into hollow or dish-shaped structures without the use of cooperating dies have been proposed in the past. For example, heat exchange plates have been made by bonding'two sheets of ductile metal together about an area and then pressurizing the unbonded area between the two sheets. The pressurizing caused the metal to be deformed outwardly beyond its elastic limit. In this system, the metal was constrained only by the surrounding welded metal and wrinkles and creases would invariably develop in .certain areas. Further, the depth of the draw was limited by the percent of elongation which the metal could undergo without rupture. Although undesirable, these limitations were tolerable in some applications.

The subject invention provides a method by which large components of the type mentioned can be fabricated rapidly and inexpensively with a minimum of tooling, The method and the apparatus allow fabricashaped structures having a length greater than their width. The method includes the steps of:

a. providing a metal sheet with a length nearly as great as the actual length of the desired structure and a width substantially as great as the width of the structure measured over its surface;

b. sealing the sheet to a fluid impervious surface continuously about an area having a width substantially as great as the surface width of the desired structure and a length slightly less than the actual length of the desired structure; and,

c. thereafter, creating a pressure differential across the sheet and within the sealed area sufficiently to deform the sheet outwardly away from the surface while simultaneously applying a tension force longitudinally of the sheet while permitting the lateral edges of the area to move toward one another.

Preferably, and in accordance with a more limited aspect of the invention, the air impervious surface is a second sheet of metal having dimensions generally corresponding to the first sheet. Also, the sealing is preferelliptically-shaped clamp bars sized so as to encircle the noted area and engage on opposite sides of the two sheets. Means, such as mechanical clamps, are provided for tightly clamping the two frame sections to the sheets to seal the space between the sheets and within the area. Also, means are provided for supplying fluid pressure within the frame and between the sheets. The

lateral sides of the frames are sufficiently flexible so that as the sheets are moved apart by the fluid pressure, the sides of the frames are pulled toward one another. Because the circumference of the frame is constant, the ends of the frame must move outwardly or apart as the sides are pulled together. The outward movement of the ends of the frame automatically applies longitudinal tension to the two sheets.

Within limits, the frame assembly can replace extremely expensivef forming dies and perform deep draw-type forming operations. The frame causes the resultant structure to have substantially noelongation of the metal in the transverse direction since the lateral sides of the frame merely move inwardly as the forming takes place.

The invention also contemplates that the inward movement of the lateral sides can be limited at various points such as by fixed stops, to influence the shape of the resulting structure.

Accordingly, a primary object of the invention is the provision of a method and apparatus particularly suited for forming dish-shaped sheet metal structures without the use of cooperating dies.

-Yet another object is the provision of a forming method of the type described wherein the metal is sub jected to little or no elongation in its transverse dimension.

A still further object is the provision of an extremely simple, flexible frame assembly which allows the method to be carried out very inexpensively.

' The above and other objects and advantages will become apparent from the following description when read in conjunction with the accompanying drawings wherein:

FIGS. 1 through 4 are pictorial views somewhat diagramatic showing the sequence of operations used for forming metal in accordance with a preferred embodiment of the invention;

FIG. 5 is a plan view, somewhat diagramatic, showing the changes which take place in the dimensional relationships of the metal sheet during the forming operation shown in the FIGS. 1 through 4 embodiment;

FIGS. 6 and 6a are cross-sectional views taken on lines 6--6 and 6a6a of FIG. 5;

gramatically. As will become apparent hereafter, this sequence of steps could vary substantially and differ from that which will be shown and described; however,-

in the preferred form of the invention, the forming pro cess begins by the provision of two sheets of metal 10 and 12, each having an extent at least sufficient to form the blank for the structural component desired. The size of the starting sheets or blanks relative to the finished component will be descirbed subsequently.

In the embodiment under consideration, the finished component to be formed is generally eliptical having a dish-shaped configuration and a length substantially greater than its width. In the FIG. 1 showing, the miniminimum length of the sheet preferably be, for example, in-the range of 14 to 16 feet long and that its width be at least'nearly 30 inches wide.

With the two sheets 10 along line 14, fluid pressure is introduced between the sheets and within the sealed area of line 14. The actual pressure required for carrying out the invention is relatively small and for the case of thin aluminum sheets of the size referred to, the pressure in the range of 5 to psi will achieve the desired results.

While the sheets are tightly sealed along line 14 and during the time the fluid pressure is introduced, substantial longitudinal tension is simultaneously applied as shown by the arrows 17 in FIG. 3. The tension gener- In FIG. 1, a desired starting configuration for the structural component which is to be formed is illustrated on the sheet 10 with the dotted line 14. The sheets 10 and 12 are illustrated as rectangular but they could be other shapes so long as they are larger than the area encompassed by line 14. Also, the sheets can be cut to the exact shape and size of the line 14. This will be illustrated with respect to the preferred form of apparatus shown in FIGS. 7 through 9.

Line 14 is empirically derived mathematically as a function of the final shape desired for the component ated must preferably be sufficient to cause the sheets to plastically deform and to be elongated to the desired final length.- The tension required can be easily calculated from the width and thickness of the sheets and known parameters of the metal being formed. The presentchange in length due to plastic deformation will be at least 5 percent when formed with the preferred apparatus but can, of course, be less or even substantially greater and up to the ultimate permissible for the metal.

During the inflation and longitudinal tensioning, the lateral edges of the sheets are constrained only slightly, or not at all and are permitted to draw toward one another as the metal deforms outwardly. They are permitted to move beyondthe desired final spacing, i.e, the width of the structure, by an amount necessary to accommodate the spring back," inherent in forming metals. Further, the edge of the area, i.e. line 14, is preferably held in a single plane and is not permitted to bend or twist.

FIG. 4 shows the two sheets at the completion of the stretching and inflating process. Note that the sheet 10 has been dished outwardly within the area of line 14. (Although not shown, sheet 12 is similarly dished.) Additionally, the sheets have been elongated in the longitudinal direction. As mentioned, during the inflating being formed. To form a hull of the type under consideration, it generally is eliptical and its total length L is only slightly less than the resulting desired length of the hull structure. Its width D is preferably substantially equal to the actual width of the hull measured over the surface. That is, D equals the surface dimension of onehalf of the hull.

In the subject embodiment, the component to be formed is one-half of an elongated, hollow aluminum hull member which is intended for use as one of the hulls of a catamaran. The complete hull is formed by dishing or forming two sheets of aluminum and joining them along mating edges.,ln FIG. 1,.sheets 10 and 12 will be simultaneously formed to each form one-half of the hull structure.

In accordance with the invention, the sheets 10 and 12 are joined together along line 14, preferably by clamping, so that they are in air tight sealed engagement along the line. FIG. 2 diagramatically illustrates clamping pressure applied to the sheets along line 14 by a series of arrows l5.

process, the lateral edges are restrained or constrained only slightly so that the transverse dimension is not stretched but permitted to move inwardly as the inflation takes place. Thus, the metal in the resulting dishshaped structure has been strained and elongated in the longitudinal dimension but substantially unstrained in the transverse dimension.

FIG. 5 illustrates the plan view of one sheet within the line 14 before and after the forming process. The solid line shows the line 14 at the completion of the forming process. As best shown in FIGS. 6 and 6a, little or no elongation takes place in the sheet transversely to its longitudinal direction. However, in the longitudinal direction, the length of the sheet has been increased substantially. The amount of longitudinal stretch which the sheet can undergo is, of course, determined by the particular type of material being formed. For example, with aluminum, elongation of 12 to 25 percent can be achieved without tearing of the sheet, depending on the particular alloy employed.

Many different shapes have been formed by use of the described method. The best results have been achieved, however, when there are no sharp corners, re'entrant angles or curves and when the length is at leasttwice the width. Further, although it is preferred to not strain the sheet in the transverse dimension, good results can be obtained with some straining.

Many different types of structures and apparatus could be used for carrying out the described method.

For example, the clamping or sealing could be accomand 12 in sealed engagement plished by pneumatically or hdyraulically actuated clamping jaws and the elongation can similarly be done through the use of fluid cylinders or mechanically. One aspect of the subject invention, however, concerns the provision of an extremely simple, flexible frame assembly which permits the internal pressures acting within the sheets to produce the forces required for sheet elongation. FIGS. 7 through 9 show a preferred form of highly simplified apparatus in which this particular technique can be carried out.

.Referring in particular to FIG. 7, the apparatus is shown as comprising a relatively flexible frame assembly 20 having an eliptical contour corresponding to the starting configuration of line 14 of FIGS. 1 through 5. The frame assembly 20 includes a pair of frame members 22 and 24 which, in the subject embodiment, are formed from steel bar bent to the eleptical shape illustrated and joined at their ends. As will become apparcm, the required strength of the bars will vary depnding upon the particular structural configuration being formed and the longitudinal compressive loads to which the bars will be subjected during a forming operation. In the embodiment shown, the bars have 1 /2 inch Xl inch cross-section throughout their entire circumference. The bars are each of identical shape and are arranged to engage and grip the sheets and 12 along the line 14.

- Many different types of gripping arrangements could be provided so that the sheets are tightly held and sealed along line 14. FIG. 9 shows a simple form of sealing and gripping jaw used in the subjec embodiment. Note that bar or frame member 22'has a small groove 26 formed continuously about its lower edge surface. Bar 24 has a similar, aligned groove 28 formed about its top edge surface and a metal key or tongue member 30 is positioned within the groove to extend outwardly a short distance. Thus, when the bars are clamped on the sheets 10 and 12, the sheets are sealed and tightly gripped by cooperation between the outwardly extending portion of strip 30 and the groove 26.

The required clamping force can be applied to the two bars 22 and 24 in many different ways. For example, air or hydraulic cylinders can be used to draw the clamp bars together. In'the embodiment under consideration, the clamping is accomplished by a large number of simple clamp assemblies 32. The clamp assemblies 32 are best shown in, FIG. 8 and each comprise a pair of L-shaped bar members 34 which engage the clamp bars 22 and 24 in the manner shown. A bolt 36 extends through aligned openings in the members 34. By tightening the nuts 38, the frame assemblies can be tightly clamped together. It is important to note that the clamp assemblies 32 are arranged so that they do not affect the flexibility of the frame assembly. That is, the lateral sides of the frame can flex inwardly during the application of fluid pressure between the sheets 10 and '12.

With the sheets tightly clamped between the frame members 22, 24, fluid pressure, preferably air, is applied between the sheets. This can be accomplished in many ways, such as, for example, merely by having a simple, quick connect fitting connected to one or the other of sheets 10 and 12, or by a small fitting extending inwardly between the two sheets from the edge. During application of fluid pressure to the space between the sheets, the sheets tend to bow outwardly, i.e.

toward and away from the viewer of FIG. 7. During this outward bowing of the sheets, the lateral sides of the frames are pulled inwardly toward one another because of the geometry of the frame and its flexibility. Simultaneously with the inward movement of the lateral sides, the end portions of the frame are driven outwardly. That is, the length of the sheets 10 and 12 must increase because the circumference of the frame remains constant while its width decreases. Thus, the frame automatically applies the required longitudinal stress while permitting the lateral edges to move inwardly substantially unconstrained.

Depending upon the final shape desired, one or the other or both of the lateral sides can be constrained at -various points during the inflation process to influence the final shape of the structure. In the subject embodiment, it is desirable that the edge 40 of the structures have a somewhat straight configuration. For this reason, the frame assembly includes a rigid beam 42 in the form of a steel channel. The frame 42 is connected to the bar 22 by a plurality of tie rods 44. The tie rods 44 have nuts 46 adjusted so that as the lateral sides of the frame sections 22 and 24 move inwardly, the nuts engage the beam at various points providing a stop to prevent further inward movement of the edge. Through the use of this arrangement, the amount of flexure which either one or both of the sides undergo can be varied. It should be appreciated that various other types of frame assemblies and motion limiting arrangements could be used for carrying out the invention. Ad

ditionally, although the length of the structures formed must be greater than their width to permit use of the in ventive method, the variations can be substantial.

The invention has been described in great detail sufficient to enable one of ordinary skill in the metal forming art to make and use the same. Obviously, modifications and alterations of the preferred embodiment will occur to others upon a reading and understanding of the specification and it is my intention to include all such modifications and alterations as part of my invention insofar as they come within the scope of the appended claims.

What is claimed is:

l. Apparatus for forming a pair of metal sheets into dish-shaped structures comprising:

a. an elongated frame assembly comprising a pair of cooperating sections, each section having spaced, elongated side members joined by shorter end members, said side members and said end members being joined to define a continuous relatively narrow planar periphery, each of said side members being flexible in directions parallel to the plane of the periphery and relatively rigid in directions perpendicular to the plane of the periphery;

b. clamp means for clamping said cooperatingsections on opposite sides of a pair of metal sheets with the continuous peripheries in alignment and I clamped toward one another to seal said sheets together along a continuous line about said area;

0. means for supplying a fluid between said sheets under sufficient pressure to deform' said sheets apart within said area; and,

(1. said clamp means carried by said pair of cooperating sections to move therewith in directions parallel to said plane of the periphery when fluid pressure is applied between the sheets.

means in directions parallel to said single plane to elongate said area while simultaneously reducing the width and maintaining the perimeter relatively constant.

3. The apparatus as defined in claim 2 wherein said means for moving said clamps comprises a somewhat elliptical frame having substantial rigidity in directions perpendicular to said plane and being flexible in directions parallel to said plane.

4. The apparatus as defined in claim 3 wherein said somewhat elliptical frame is comprised of two sections. 

1. Apparatus for forming a pair of metal sheets into dish-shaped structures comprising: a. an elongated frame assembly comprising a pair of cooperating sections, each section having spaced, elongated side members joined by shorter end members, said side members and said end members being joined to define a continuous relatively narrow planar periphery, each of said side members being flexible in directions parallel to the plane of the periphery and relatively rigid in directions perpendicular to the plane of the periphery; b. clamp means for clamping said cooperating sections on opposite sides of a pair of metal sheets with the continuous peripheries in alignment and clamped toward one another to seal said sheets together along a continuous line about said area; c. means for supplying a fluid between said sheets under sufficient pressure to deform said sheets apart within said area; and, d. said clamp means carried by said pair of cooperating sections to move therewith in directions parallel to said plane of the periphery when fluid pressure is applied between the sheets.
 2. Apparatus for forming dish-shaped sheet metal structures having a length substantially greater than their width from a pair of metal sheets comprising: a. clamp means positioned along a continuous line lying substantially in a single plane encircling an area having a length substantially greater than its width for fluid sealing a pair of said sheets together in face-to-face relationship along the line; b. means for supplying fluid pressure between said sheets and within said line; and, c. means operable in response to the introduction of pressure within said area for moving said clamp means in directions parallel to said single plane to elongate said area while simultaneously reducing the width and maintaining the perimeter relatively constant.
 3. The apparatus as defined in claim 2 wherein said means for moving said clamps comprises a somewhat elliptical frame having substantial rigidity in directions perpendicular to said plane and being flexible in directions parallel to said plane.
 4. The apparatus as defined in claim 3 wherein said somewhat elliptical frame is comprised of two sections. 